diff --git a/sys/sys/mbuf.h b/sys/sys/mbuf.h
index fc5711dc1122..4e6e59ebccde 100644
--- a/sys/sys/mbuf.h
+++ b/sys/sys/mbuf.h
@@ -67,8 +67,10 @@
  * type:
  *
  * mtod(m, t)	-- Convert mbuf pointer to data pointer of correct type.
+ * mtodo(m, o) -- Same as above but with offset 'o' into data.
  */
 #define	mtod(m, t)	((t)((m)->m_data))
+#define	mtodo(m, o)	((void *)(((m)->m_data) + (o)))
 
 /*
  * Argument structure passed to UMA routines during mbuf and packet